By casting magic jar, the character places the character's own soul in a gem or large crystal (known as the magic jar), leaving the character's body lifeless. Then the character can attempt to take control of a nearby body, forcing its soul into the magic jar. The character may move back to the jar (returning the trapped soul to its body) and attempt to possess another body. The spell ends when the character sends the character's soul back to the character's own body (leaving the receptacle empty).

To cast the spell, the magic jar must be within spell range and the character must know where it is, though the character does not need line of sight or effect to it. When the character transfers the character's soul upon casting, the character's body is, as near as anyone can tell, dead.

While in the magic jar, the character can sense and attack any life force within 10 feet per caster level (on the same plane). The character does need line of effect from the jar to the creatures. The character, however, cannot determine the exact creature types or positions of these creatures. In a group of life forces, the character can sense a difference of four or more HD and can determine whether a life force is positive or negative energy. (Undead creatures are powered by negative energy. Only sentient undead creatures have, or are, souls.)

Attempting to possess a body is a full-round action. It is blocked by protection from evil or a similar ward. The character possesses the body and forces the creature’s soul into the magic jar unless the subject succeeds at a Will save. Failure to take over the host leaves the character's life force in the magic jar, and the target automatically succeeds at further saving throws if the character attempts to possess its body again.

If successful, the character's life force occupies the host body, and the host’s life force is imprisoned in the magic jar. The character keeps most mental abilities and gains some physical abilities, as with polymorph other (except that the character gets the creature’s actual physical abilities, not average ones).

As a standard action, the character can shift freely from a host to the magic jar if within range, sending the trapped soul back to its body. The spell ends when the character shifts from the jar to the character's own body.

If the host body is slain, the character returns to the magic jar, if within range, and the life force of the host departs (that is, it is dead). If the host body is slain beyond the range of the spell, both the character and the host die. Any life force with nowhere to go is treated as slain.

If the spell ends while the character is in the magic jar, the character returns to the character's body (or dies if the character's body is out of range or destroyed). If the spell ends while the character is in a host, the character returns to the character's body (or dies, if it is out of range of the character's current position), and the soul in the magic jar returns to its body (or dies if it is out of range). Destroying the receptacle ends the spell, and the spell can be dispelled at either the magic jar or the host.

Incorporeal creatures with the magic jar ability can use a handy, nearby object (not just a gem or crystal) as the magic jar.